1. Field of the Invention
This invention relates to machinery and more particularly to ball screw assemblies having limit stops.
2. Prior Art
Ball screw assemblies are frequently employed in machinery for the conversion of rotational movement to linear movement. Such assemblies include, generally, a shaft member having an outer diameter helical screw groove and a ball nut received around the shaft member with balls seated in the screw groove such that rotation of the shaft member causes axial advancement of the ball nut along the shaft. With increases in the accuracy of machining of the screw, together with advances in controlability of the rotational input to the screw, such assemblies have found wide acceptance in various machinery designs including designs where accuracy of linear positioning is critical. In many such designs, due to the use of fine pitched threads coupled with high speed rotational input, serious adverse consequences can occur if over travel of the ball nut is not prevented. However, the use of fixed limit stops absolutely precluding over travel can cause shock damage to the machinery if the ball screw contacts the limit stop at an appreciable rate of linear advance.
For this reason, it has been suggested in the art to employ yieldable limit stops either by the use of springs or elastomeric compression members. See for example, U.S. Pat. No. 3,718,215 which illustrates a device employing springs and U.S. Pat. Nos. 3,732,744 and 3,762,277 for devices employing elastomers.
While such prior suggested constructions have the ability to retard axial movement of the ball nut, they do not act directly on the ball screw which, in most installations, is the drive source for the ball nut. To the extent that rotation of the ball screw is not retarded, resistance to axial movement of the ball nut provides heightened stresses at the ball screw/ball nut interface. It would therefore be an advance in the art to provide for both yielding resistance to axial movement of the ball nut and resistance to rotation of the ball screw. It would be a further advance in the art to provide such resistances in a complementary fashion where the resistance to continued movement of each of the ball nut and ball screw is interrelated.